Q: How many reps per Ab Exercise should I do?

Answer: To bulk up, use more weight and less than 12 repetitions. To make your abs look hard, use body weight and more than 20 reps per set

As with any other muscle, you need resistance to put on more mass. Besides cutting down body fat until your abs show up, some muscles need to be built up. If you're favored by genetics, there are still some pretty good looking abs beneath your body fat.

So to build up more massive muscles, use weights to increase resistance and do 6-12 reps per set. To keep your belly flat, but not to massive, do sets beyond 20 repetitions. Always watch your progress and react accordingly.

Q: What is the Best Ab Workout?

Answer: The Best Ab Workout for the whole abdominal part would be Crunches.

Try to train your abs from different angles. Do Leg Raises for your lower abs, Side Bends for your obliques (also see video below) and Crunches for the big picture.

Q: Should I do Ab Exercises at the beginning or at the end of my Workout?

Answer: Do Ab Exercises after your main workout and before your cardio workout.

There is almost not one single body part exercise where your abdominal muscles do not provide support or stabilize your whole body. Therefore a careful warm-up of your ab muscles before starting your actual workout (chest, back, ...) could be applied. But don't understand this warm-up as an intense ab workout.

Just because of the fact that your ab muscles have to support other muscles doing for example back exercises, they shouldn't be to exhausted. Your actual ab exercises should follow after your regular workout (and perhaps before your final cardio exercises).